SUMMARY: 1) An endogenous inhibitor of cyclic nucleotide phosphodiesterase (PDE) from bovine brain was purified and identified by thin layer chromatography and HPLC as hypoxanthine. Modes of action of hypoxanthine, inosine, inosine 5'-monophosphate (IMP) on PDE from bovine brain were elucidated. 2) The supernatant of the cultured human leukemia cells, CEM, was fractionated on a DEAE-cellulose column into two PDEs, PI and PII, in the order of elution from the column. In the presence of free Ca(II), the PI PDE was activated threefold by calmodulin (Ca(II)-dependent protein activator). Calmodulin has no effect on PII enzyme. Both PI and PII PDEs can hydrolize either cyclic AMP or cyclic GMP. PII enzyme was inhibited by hypoxanthine, inosine, IMP, theophylline or 3-isobutyl-1-methyl xanthine (IBMX). The mechanisms of inhibition were also studied. 3) A switch in Ca(II)-dependent to Ca(II)-independent cyclic GMP PDE fails to occur during development in retinas of Irish setter dogs with inherited degeneration. This coupled with low levels of calmodulin in affected retinas appears to result in the derangement in cyclic nucleotide metabolism that is characteristic of this hereditary degenerative disease.